This invention relates to a hetero-structure semiconductor laser which is formed on an InP substrate.
With the progress of studies for the reduction of a loss in optical fibers made of silica, the loss has now been made small in the vicinity of 1.55 .mu.m to such an extent as to be considered ultimate at the present stage; and this wavelength range is considered optimum for a long-distance optical fiber transmission. Light sources for use in the abovesaid wavelength range have been developed by laying stress on an InGaAsP laser formed on an InP substrate, but there has not yet been obtained a light source which performs the CW operation (a continuous operation) at room temperature. The reason for this is that it is difficult to obtain a good quality crystal having a multi-layer, hetero structure, although such crystal has been available for use at shorter wavelengths.